Integrated circuits (ICs), used in a variety of applications, generally include high speed transceiver blocks. High speed receivers and transceivers are used in system on chip (SOC) applications that require a wide frequency tuning range. An inductance-capacitance voltage-controlled oscillator (LC-VCO) is generally used to control the tuning frequency of transceiver blocks in an IC. The tuning range of the LC-VCO consequently determines the tuning range of the transceivers in the IC.
An LC-VCO may be constructed with an inductor (L) and a variable capacitor (C), formed by one or more varactors. For example, the variable capacitor may be a pair of varactors coupled to a control voltage from a charge pump, where the capacitance is controlled by varying the control voltage. The inductor and variable capacitor form an inductor-capacitor (LC) parallel resonance tank. As the capacitance of the variable capacitor (C) is controlled by the control voltage, the oscillating frequency, fosc, of the LC parallel resonance tank may be represented by the following equation:
      f          osc      ,      min        <      f    osc    <            f              osc        ,        max              ⁢                  ⁢    or    ⁢                  ⁢          1                        LC                      var            ,            max                                <      2    ⁢                  ⁢    π    *          f      osc        <      1                  LC                  var          ,          min                    
Thus, based on the above equation, the capacitance of the LC tank is inversely proportional to the oscillating frequency. In other words, increasing the capacitance decreases the minimum and maximum oscillating frequencies, and decreasing the capacitance increases the minimum and maximum frequencies. As such, to achieve a maximum frequency tuning range, the capacitance needs to be fully controllable. In general, the inductance (L) may be fixed and the capacitance (C) is controlled by a control voltage. An additional capacitor may be incorporated into the circuit in order to lower the oscillating frequency. However, as the additional capacitor is generally not coupled to the control voltage, the change in the oscillating frequency may not be fully adjustable at the lower frequency range.
Therefore, in order to achieve a wider tuning range, it is desirable to be able to control or tune all the capacitors in the LC tank in order to tune the circuit even when the circuit is operating at a lower frequency. It is within this context that the invention arises.